To reignite something is to light it on fire again. If you want to reignite your campfire after pouring a pail of water on it, you'll have to find some dry wood.
You can literally reignite things like gas stoves, birthday candles, or sparklers if their flames have been extinguished. You can also figuratively reignite something by making it flare up in some way: "The release of the balloons at the end of the rally was sure to reignite the crowd's excitement." This word adds the "again" prefix re- to ignite, from the Latin root ignis, "fire."